As shown in FIG. 1, integrated circuits are typically encapsulated in a package 10 of a suitable material, such as epoxy, from which conductive leads 12 project. Although the leads 12 are shown in FIG. 1 as being conductors that project laterally and then downwardly, other lead configurations are in common use.
The encapsulation of the integrated circuit is typically performed by placing the integrated circuit in a mold then injecting a mold compound into the mold. A typical integrated circuit injection mold 20 is shown in FIG. 2. The mold 20 includes a rectangular upper mold section 26 and a matching lower mold section 28, each of which have a series of mold cavities 36, 38, respectively, formed in respective adjoining mold surfaces 30, 32. Each of the adjoining pairs of mold cavities 36, 38 generally encapsulate a single integrated circuit, although it is possible to encapsulate several interconnected integrated circuits in a single mold cavity pair. Eight mold cavities 36, 38 are shown in FIG. 2, but a greater or lesser number of mold cavities may be formed in conventional mold sections. The mold cavities 36, 38 are typically rectangular to match the desired shape of the integrated circuit package 10 (FIG. 1), but other shapes are also possible.
In practice, before the integrated circuits are placed in respective pairs of mold cavities 36, 38, they are attached to a leadframe, and the integrated circuit and leadframe are placed between the mold sections 26, 28. A typical leadframe 40 is shown in FIG. 3A. The leadframe 40 includes several leadframe sections 44 corresponding in number to the number of pairs of mold cavities 36, 38, and corresponding in size and shape to the size and shape of the mold cavities 36, 38. A single leadframe section 44 is shown in FIG. 3B. With reference to FIG. 3B, each leadframe section 44 includes a central mounting plate 46 to which an integrated circuit 48 is mounted by suitable means, such as adhesive tape (not shown). Each leadframe section 44 also includes a plurality of inner leads 50 projecting from each side of the central mounting plate 46 to a respective gasket strip 52, and a plurality of outer leads 56 aligned with respective inner leads 50 extending from the gasket strips 52 to an outer strip 58. The outer leads 56 are what eventually form the leads 12 shown in FIG. 1. The leadframe 40 also includes leadframe rails 60, 62 extending along the longitudinal edges of the leadframe 40. As explained in greater detail below, after the package 10 has been formed, the portion of the leadframe 40 extending beyond the outer leads 56 and the leadframe rails 60, 62 are removed. As a result, the only portion of the leadframe 40 extending beyond the package 10 are the outer leads 56 (i.e., 12), as shown in FIG. 1.
The leadframe 40 to which the integrated circuit 48 is attached is placed in the mold 20 between the mold sections 26, 28, with each leadframe section 44 aligned with a respective pair of mold cavities 36, 38. The leadframe 40 is also placed in the mold 20 so that the gasket strips 52 and leadframe rails 60, 62 extend around the cavities 36, 38 in contact with the mold surfaces 30, 32. As a result, the leadframe 40 acts as a gasket to retain material within the cavities 36, 38.
After the leadframe and integrated circuit 48 have been placed in the mold 20, a mold compound is injected into each pair of the cavities 36, 38 through a respective injection inlet 70 (FIG. 2) provided for each pair of mold cavities 36, 38 at one edge thereof. The injection inlets are formed in either or both of the mold sections 26, 28. The injection inlets 70 provide a path for the mold compound, generally an epoxy compound, to be injected into the mold cavities 26, 28. The mold compound attempts to displace air in the mold cavities 36, 38, and this air must therefore be vented from the cavities 36, 38. For this purpose, mold vents 74 are formed in either or both of the mold sections 26, 28 through an edge of each pair of mold cavities 36, 38 opposite the injection inlets 70.
After the mold compound has solidified, the mold sections 26, 28 are separated to allow the leadframe 40 and packages 10 to be removed from the mold 20. As previously explained, the leadframe 40 forms a gasket between the mold sections 26, 28 to prevent the mold compound from leaking out of the mold cavities 36, 38. In practice, however, some of the mold compound leaks past the gasket strip 52 onto the outer leads 56 and the outer rails 58. Some of the mold compound also leaks onto the leadframe rails 60, 62. The undesired mold compound on these exposed portions of the leadframe 40 is known as mold compound “flash.”
Throughout the manufacturing process, the leadframe 40 may be exposed to various contaminants, such as the mold compound flash, as described above. These contaminants, including mold compound flash, are removed from the leadframe 40 in a deflashing procedure after the leadframe 40 and packages 10 have been removed from the mold 20. The deflashing procedure also cleans the leadframe 40 so the leadframe 40 can be properly electroplated, as described below.
Several varieties of deflashing procedures may be used. One conventional deflashing procedure uses an electrolytic deflash machine (not shown) that consists of two main components, an electrolytic deflash section and a high pressure rinse section. The electrolytic deflash section includes a deflash cell in the form of a box that is lined with iridium oxide coated titanium plates. The plates become anodes when they are connected to a positive voltage. An electrolyte is pumped into the deflash cell from a mixing tank to immerse the plates. A stainless steel conveyor passes through the deflash cell just over the electrolyte. The conveyor becomes the cathode when it is connected to the negative voltage through copper brushes. The leadframes 40 are suspended vertically from the conveyor so that they are immersed in the electrolyte. An electrical current is passed through the electrolyte to ionize some of the water in the deflash solution, thereby creating positively charged and negatively charged ions. The positively charged ions (H+) migrate to the negatively charged leadframes 40. When the hydrogen ions reach the surface of the leadframes 40, they gain an electron and join together to create hydrogen gas bubbles. The rapid evolution of hydrogen bubbles on the surface of the leadframes 40 causes sufficient agitation to loosen the surface contamination. After the rapid evolution of hydrogen bubbles loosens the surface contamination, the leadframes 40 are transported to the high pressure rinse section where they are carried through a high pressure water rinse. The high pressure rinse blasts away the loosened surface contamination from the leadframes 40.
Although the electrolytic deflash procedure is described in some detail herein, other conventional deflash procedures may be used. For example, the leadframe 40 may be sprayed with abrasive particles in a process similar to sandblasting. The mold compound flash may alternatively be removed from the leadframe 40 by high intensity laser radiation or some other means.
After the deflashing procedure, the leadframe 40 is typically electroplated with a lead/tin alloy. The alloy prevents oxidation of the leadframe 40 and promotes the solderability of the leadframe 40. The leadframe 40 may be fabricated from copper, for example, and oxidation would increase its contact resistance if the leadframe 40 was not electroplated.
Regardless of the deflashing procedure that is used, the deflashing procedure often fails to adequately remove the flash from the leadframe 40. Instead, although some of the flash is removed, an unacceptable amount commonly adheres to the leadframe 40, particularly on the leadframe rails 60, 62, which are the exposed surfaces of the leadframe 40 having the largest area. This flash remaining on the leadframe 40 can prevent the leadframe 40 from being properly electroplated. Also, the remaining flash can cause a number of problems in subsequent productions stages, as explained below.
After undergoing the deflashing and electroplating procedures, the leadframe 40 undergoes a trimming and forming procedure. This procedure is generally accomplished using a trimming and forming tool that cuts the outer rails 58 and leadframe rails 60, 62 from the leadframe 40, thereby leaving only the outer leads 56 as exposed portions of the leadframe. In the forming procedure, the outer leads 56 are bent to the configuration of leads 12 shown in FIG. 1.
The final manufacturing procedure is a visual inspection of the package 10 and leads 12. This procedure is generally performed using a vision system, such as a video camera coupled to a computer that has been programmed with software to examine the package and leads 12 for such parameters as lead spacing and bending form.
As mentioned previously, an unacceptable amount of mold compound flash commonly remains on the leadframe 40 after the deflashing procedure. This flash can flake off from the leadframe 40 during the trimming and forming procedure, resulting in debris that can interfere with the operation of the trimming and forming tool used to perform the trimming and forming procedure. It is therefore necessary to frequently clean the flash debris from the trimming and forming tool, which requires manpower and takes the trimming and forming tool out of service for the period of time needed to complete the cleaning operation. The flash debris can also increase the maintenance required for the trimming and forming tool and shorten its useful life. A significant amount of flash debris can also remain on the package 10 and leads 12 after the trimming and forming procedure, thereby interfering with the operation of the vision system that inspects the package 10 and leads 12. For example, flakes of flash debris finding their way onto a lens of the vision system may obscure the view of the package 10 and leads 12. Flash debris flakes may also cause false inspection results that may cause unwarranted rejection of an integrated circuit. For example, a flake of flash debris on a lead 12 may cause the vision system to interpret adjacent leads to be too closely spaced to each other.
There is therefore a need for a device and method that substantially prevents mold compound flash debris form interfering with subsequent processing steps.